Pharmaceuticals, reagents, diagnostic tests, and other objects often require refrigerated storage for preservation. Exposure to conditions below recommended temperatures, usually the freezing point of an aqueous solution, may change the operating characteristics of the object or render the object useless for its intended purpose. For this reason, many objects are packaged with a critical temperature indicator, which alerts the end user if the object has been exposed to conditions below the recommended critical temperature.
One known type of critical temperature indicator is disclosed by Manske, U.S. Pat. No. 4,457,252,l incorporated herein by reference. Manske discloses a critical temperature indicator having a thermometer-like bulb and capillary tube housing. The bulb and a portion of the capillary tube are filled at ambient temperature with a first colorless reagent, which is separated within the capillary tube from a second dyed reagent by a very small amount of liquid or (barrier segment), which is not miscible with either of the reagents. In the preferred embodiment, the reagents are organic compounds. At the solidification temperature, the first reagent in the bulb undergoes a significant volume reduction, thereby drawing by capillary action the barrier segment and the second dyed reagent into the bulb. The two reagents mix and the bulb assumes the color of the dye. Because the reagents do not separate upon heating, the device gives an irreversible indication that the device has been exposed to the solidification temperature of the first reagent, which is also the critical temperature of the device.
The housing of Manske's device is formed from a continuous piece of capillary tubing. While the bulbous end of the tube is closed, the other end is open to the atmosphere. As a result, Manske's device often fails or prematurely “indicates” due to evaporation of the barrier segment through the open end of the capillary tube.
Another cause of failure of Manske's device is barrier segmentation. It is known that normal temperature fluctuations below the critical temperature cause movement or “cycling” of the reagent fluids and barrier segment back and forth within the glass capillary tube. During cycling, the barrier segment and reagents adhere to minute “nucleating” sites present on the capillary tube. Over time, the volume of fluid adhered at the nucleating sites steadily increases, which causes segmentation of the barrier segment and premature indication of the device. Both barrier segment evaporation and segmentation decrease the usable shelf life of the device.